1. Field of the Invention
The invention relates to circuits implementing cells containing resistors and capacitors, more precisely resistor/capacitor couples. More particularly, the invention relates to the adjustment of RC cell time constants within integrated circuits.
2. Description of the Related Art
Currently, a great number of electronic circuits are fabricated as integrated circuits. Some of these electronic circuits use RC cells, in other words resistor/capacitor couples, for performing filtering or timing functions, for example.
By way of example, FIG. 1 shows an integrated circuit 1 incorporating a DC/DC converter designed to transform a battery voltage VBAT into a regulated power supply voltage VCC for powering a circuit represented by the impedance ZL.
The principle implemented by this DC/DC converter is known per se. The battery voltage is chopped by the pair of transistors 10 and 11 controlled by a rectangular signal with a modulated duty cycle. The inductance LR and the capacitor CRprovide the filtering for this chopped voltage in order to have a DC power supply voltage VCC. An error amplifier 12 amplifies the error between the power supply voltage VCC and a reference voltage VRef. A ramp generator 13 supplies a periodic signal in the form of a ramp. A comparator 14 compares the amplified error with the periodic ramp signal. The signal delivered by the comparator 14 corresponds to the rectangular signal with variable duty cycle that controls the pair of transistors. The modulation of the duty cycle depends on the error between the reference and power supply voltages, thus creating a feedback loop that stabilizes the power supply voltage VCC.
The DC/DC converter comprises several RC cells. The error amplifier 12 also provides a low-pass filtering function comprising two poles and two zeros in order to stabilize the feedback loop operation. However, the feedback loop must provide a high reactivity, in other words a cut-off frequency that is as high as possible in order to rapidly compensate for a variation in the battery voltage VBAT or in the load ZL so as to ensure that the DC supply voltage VCC remains constant. The ramp generator 13 can comprise one or more RC cells. The frequency and the form of the periodic ramp signal depends on the time constant of these RC cells.
The precision on the variations in the resistor and capacitor values of the RC cells depends on an accumulation of tolerances associated with the fabrication method of the integrated circuit (notably fabrication mask alignment defects, dispersions in the dopant concentration, thickness tolerances of the layers forming the integrated circuit) and with the operating conditions (notably, power supply voltage, temperature). Currently, the tolerance in the resistor and capacitor values is 35% and 20%, respectively. When an RC cell is produced, the tolerances in the values are cumulative and the resulting tolerance in the time constant is 62%.
Several techniques are known to pre-empt this problem of tolerances. A first technique consists in choosing the circuit dimensions so that it can operate in the worst-case situation. In the example of the DC/DC converter, this means slowing down the regulation loop, in other words reducing the circuit bandwidth. This leads to a loss of performance in terms of regulation. The power supply voltage VCC will take more time to recover to the level of VRef.
A second technique consists in installing passive components external to the integrated circuit so that these can be precisely chosen and/or measured in order to know their exact values. This technique goes counter to achieving the maximum integration of the circuits. It is costly since it requires the addition of external components. In general, it is only used for high-precision circuits or when the passive components cannot be fabricated within the integrated circuit.
A third technique consists in adding compensation elements that counterbalance the drift in the components. This technique is reasonably well controlled but does not simply allow one compensation for all the error factors to be obtained. In the case of the error amplifier 12, the compensation for the resistors R1, R2 and R3 and for the capacitors C1 and C2 is very complex to implement.
A fourth technique involves the use of different circuits that are free of variations. In the case of an amplifier including filtering means, this technique requires a digital circuit that is very complex and very costly in terms of size of silicon chip.
And finally, a fifth technique consists in adjusting the components. The adjustment can be effected by laser during the fabrication method to compensate for the component errors associated with the fabrication method. The adjustment can be made dynamically when the circuit is powered up or during operation. This generally requires the measurement of one or more circuit characteristics, the comparison of the measurements with reference values, then the adjustment of one or more components as a function of this comparison. The adjustment can be made in a single step by means of complex computational means or by means of successive iterations.
The transfer function H(p) of the error amplifier 12 is as follows:
      H    ⁡          (      p      )        =                    (                  1          +                                    (                                                R                  1                                +                                  R                  2                                            )                        ⁢                          C              1                        ⁢            p                          )            ⁢              (                  1          +                                    R              3                        ⁢                          C              2                        ⁢            p                          )                            (                  1          +                                    R              1                        ⁢                          C              1                        ⁢            p                          )            ⁢              R        2            ⁢              C        2            ⁢      p      
The tolerance errors on the resistors R1, R2 and R3 and the capacitors C1 and C2 distort the transfer function H(p) from an ideal characteristic. The adjustment of the components requires several measurement points in order to obtain an approximation of the transfer function and significant computational means for determining from the measured characteristic the corrections to be made to the passive components.
In summary, there does not currently exist any technique for providing a simple correction of the error due to the components in the error amplifier 12 even though the latter is an amplifier equipped with relatively simple filtering means.